> While I would agree in principle, in practice we have little control
> over what customers use.
You won't have a good time at Disneyland if you ride Space Mountain in the unsupported configuration of 'not belted in'. An ISP has no control over what I set my MTU to, and they won't support me if I change it.
> Users will migrate to IPv6 in their own time and their own way.
An object at rest tends to remain at rest until outside force is applied.
The lifetime of many of the IPv4 SOHO NAT devices out there exceeds both the exhaustion timer and the most optimistic depletion estimates for the RIRs. For most people, generally only a failed NAT device comprises the outside force required to buy new equipment. Most users won't actively migrate to IPv6 in their own time and their own way; the ISP needs to take an active stance that starts with "we're only supporting IPv4/6 devices starting $date", and ends with "we're terminating all legacy IPv4 support on Jan 1, 2350 at 1:30PM.'
Nathan